Add Dockerfile and docker-compose.yml to run next to the Palladium node

ElectrumX is built from this source tree with RocksDB and joins the external
'palladium' network of the node in palladiumcore/docker, reaching it as
palladiumd:2332. RPC credentials come from .env (ignored by git, see
.env.example). The Bitcoin-specific contrib/Dockerfile is removed.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

.env.example

@@ -0,0 +1,7 @@
+# Copy to .env (never commit it). RPC credentials of the node: rpcuser/rpcpassword (or an rpcauth user)
+# in ~/.palladium/palladium.conf.
+PLM_RPC_USER=username
+PLM_RPC_PASSWORD=change-me
+#CACHE_MB=1200
+#REPORT_SERVICES=tcp://electrum.example.com:50001
+#ELECTRUMX_TCP_PORT=50001